Passive sensors (for implanting into the human body or for mounting at some inaccessible location within a machine) are known in the art. These sensors are typically electromagnetic, providing an electromagnetic signal when activated.
The prior art sensor system typically comprises a sensor, implanted into the machine, and an activating and detecting system. The sensor is typically an oscillating circuit whose vibration frequency changes in response to the physical variable to be measured. The oscillating circuit typically includes a capacitor and an inductor, one of which is built to vary in accordance with the physical variable being measured. As a result, the vibration frequency of the circuit is a function of the physical variable.
When the sensor is irradiated with electromagnetic energy from the activating system, some of the energy is absorbed by the oscillating circuit, depending on how close the incident frequency or frequencies are to the resonant frequency of the circuit (which, in turn, depends on the physical variable being measured). The change in the electromagnetic field due to the absorption of energy by the oscillating circuit is detected by the detecting system.
Electromagnetic sensors and systems are described in the U.S. Pat. No. 4,127,110 and in the article:
Carter C. Collins, "Miniature Passive Pressure Transensor for Implanting in the Eye", IEEE Transactions on Bio-Medical Engineering, Vol. BME-14, No. 2, April 1967.
Unfortunately, within living tissue, the passive sensor is detectable within a range no larger than 10 times the diameter of its antenna (part of the oscillating circuit). Furthermore, the sensor system is not operative within a conductive enclosure.